1. Field of the Invention
The present invention relates generally to liquid crystals, and, more particularly, to an improved configuration in a liquid crystal light valve which includes a new spacing technique for uniform and controllable liquid crystal layer thicknesses.
2. Description of Related Art
One type of liquid crystal electro-optical device is constructed by placing a thin layer of liquid crystal between two transparent plates, or one transparent plate and one mirrored plate, that have each been coated with a layer of an electrical conductor on its interior face. When no electric field is applied (field-OFF) between the conductive layers, the director of the liquid crystal is in one state. (The "director" of the liquid crystal is the macroscopic direction of the long molecular axis of the liquid crystal molecules.) When an electric field is applied (field-ON), the director reorients to another state. Because the liquid crystal is birefringent, the two states will have different refractive indices. The movement of the director causes a polarization re-orientation as the light passes through the liquid crystal layer. The state change responsive to an applied electric field is the basis for liquid crystal devices that control light, such as displays and projectors.
In its usual form, a liquid crystal light valve (LCLV) is a device that modulates a polarized projection light beam on a pixel-by-pixel basis as it passes through a liquid crystal layer. A photoactivated LCLV performs the pixelized modulation with a writing beam directed against the backside of a reflective-mode liquid crystal layer.
A simplified version of a photoactivated LCLV includes a transparent input substrate, usually comprising glass, upon which is formed a transparent back electrode layer, such as indium tin oxide or P.sup.++ semiconductor, and a layer of photoconductor material, such as silicon or cadmium sulfide. A light-blocking layer, such as SiO.sub.2 or CdTe, prevents light entering the readout side of the device from entering the photoconductor, while a dielectric or metal matrix mirror on the readout side of the light-blocking layer reflects a readout beam. A liquid crystal layer is sandwiched between alignment layers on the readout side of the mirror, with a counter-electrode layer and a front transparent substrate formed in turn on the readout side of the liquid crystal cell.
An AC voltage source is connected across the back electrode and counter electrode to establish a bias that sets an operating point for the liquid crystal. In operation, an input image from an optical source such as a cathode ray tube (CRT), a scanning laser, or the like is applied to the input side of the LCLV, while a linearly polarized readout beam is transmitted through the LC cell and reflected back from the mirror through a crossed polarizer. The input image produces a corresponding spatial voltage distribution across the LC layer, altering the localized alignment of the liquid crystal in accordance with the applied voltage pattern. This results in a spatial modulation of the readout beam, permitting a transfer of information from the input image to the readout beam.
The operation of this and other types of liquid crystal light valves is discussed in greater detail in numerous technical publications; see, for example, "Progress in Liquid Crystal Light Valves", by W. P. Bleha, in Laser Focus/Electro-Optics, October 1983, pages 111-120.
Previous versions of liquid crystal light valves (LCLVS) had relatively poor LC layer thickness uniformity, but due to the slower response and &gt;4 .mu.m thick liquid crystal (LC) layers, spacer pads on the perimeter of the display satisfied operational requirements.
Proposed liquid crystal light valves operating at TV-rates will have an LC thickness in the range of 3 to 4 .mu.m. A desire to reduce the size of the LCLV projector has resulted in high temperatures that may cause the substrates of the LCLV to bend. For these reasons, it has become necessary to place spacers into the LC layer throughout the viewing area of the display. A fast, inexpensive, and dependable method for placing these spacers into the display is an important factor in the success of the commercial venture.
Presently, spacer particles, such as glass fibers or spheres, are employed. See, for example, K. Shimizu, et al, "Optical Display Cell of Even Thickness Using Adhesive Means and Separate Adhesive-Free Spacers," U.S. Pat. No. 4,390,245, issued Jun. 28, 1983.
However, spacer particles tend to cluster, resulting in overcrowding in some regions, which reduces the active area of the display, while leaving other regions depleted, causing a possible thickness variation in the display. Depositing spacer particles in a consistently uniform manner can be very expensive. In these techniques, dust particles are also trapped on the substrate surface along with the spacer particles. Finally, in order to safely maintain a uniform separation between two substrates, too many spacers are often deposited, resulting in a poor display or device. Machines to perform this task with reasonable repeatability are very expensive.
Peripheral spacers, which have been used in the prior art, are not sufficient means to maintain uniform separation when the substrates bend.
Thus, there remains a need for a means for spacing the substrates employed in liquid crystal display cells a fixed distance without substantially interfering with display properties.